Auto white balance adjusting serves in an electronic still camera or a video camera to reproduce whiteness in an image of a white subject. In a current auto white balance adjusting system, the balance of the RGB components of each pixel signal is adjusted so that an average of the signals totally represents an achromatic color. When an image, most of which is composed of chromatic colors, is treated using this white balance adjusting system, an error in white balance adjusting, called color failure, easily occurs. Several white balance adjusting systems in which the color failure can be prevented have been proposed. Japanese Patent Laid-Open Publication No. Hei 5-292533 discloses a white balance adjusting system with a color failure preventing function. In this system, a screen is divided into a plurality of blocks. A representative value for each block of the image signal is obtained. An average of the representative values for the blocks belonging to a predetermined area of the screen is calculated. A white balance adjusting signal which represents an achromatic color in compensation for the average is obtained. The color failure can be prevented when the white balance adjusting based on this white balance adjusting signal is carried out. Japanese Patent Laid-Open Publication No. Hei 5-7369 discloses another white balance adjusting system. In this system, the range in which the white balance adjusting signal varies is limited. Overadjusting of the white balance can be prevented when the white balance adjusting based on this limited white balance adjusting signal is carried out.
A greenish image is usually reproduced for a white subject when a white subject is illuminated with a fluorescent lamp in a room. It is difficult to discriminate the greenish image from a green image of grass illuminated with sunlight outdoors. These images easily suffer the color failure. A video camera and electronic still camera are thought to be often used indoors in light from a fluorescent lamp or outdoors for taking pictures with a background of grass. The easy occurrence of color failure is undesirable. In the white balance adjusting system of Japanese Patent Laid-Open Publication No. Hei 5-7369 described before, it is determined by the luminance of a subject whether the subject is located outdoors or indoors. The range in which the white balance adjusting signal varies is set corresponding to the respective conditions to restrain the color failure.
The above described white balance adjusting system can appropriately adjust the white balance for the subject irradiated by predetermined light sources such as the sun, and light from fluorescent and tungsten lamps, but often cannot appropriately adjust for a subject irradiated simultaneously by a plurality of light sources or by an unidentified light source.
Although various improvements are made in the current white balance adjusting systems to prevent the color failure, the color failure occasionally occurs in adjusting the white balance for a chromatic image occupying a large part of a screen.
The subject to be imaged determines the necessity of the white balance adjusting. For example, in taking pictures in a sunset, the sky itself is properly imaged without the white balance adjusting, whereas subjects except the sky in the sunset are properly imaged using the white balance adjusting. However, the current auto white balance systems automatically operate, irrespective of the subjects to be imaged.